staging: vt6656: set usb_set_intfdata on driver fail.

intfdata will contain stale pointer when the device is detached after
failed initialization when referenced in vt6656_disconnect

Provide driver access to it here and NULL it.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/6de448d7-d833-ef2e-dd7b-3ef9992fee0e@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>